This invention relates to a variable venturi carburetor in an internal combustion engine adapted to prevent self-oscillation of a suction piston installed in the carburetor which self-oscillation will, in turn, cause surge of an automotive body as during acceleration with a throttle valve fully opened at middle and high intake air flowing stages.
Generally in a variable venturi carburetor, as the amount of reciprocating motion of a suction piston is determined by the intake air vacuum at the venturi portion or the throat portion of the associated air intake flow line, the engine power is apt to be influenced by fluctuations in the intake air vacuum. Particularly at high intake air flowing stage as during acceleration with the throttle valve fully opened, a slight level of pressure fluctuations or pulsations generated at the engine or air intake system causes self-oscillation of the suction piston and influences the amount of intake air and fuel flow, thus resulting in surge of the automotive body and unsmooth acceleration performance of the engine.
To prevent undue self-oscillation of the suction piston, an oil damper is conventionally included in the suction chamber. However, in such an oil damper, difference in viscosity of oil and variation in viscosity due to change in temperature tend to arise, thus resulting in unsatisfactory repeatability upon accurate metering of fuel flow in the carburetor. In case of employment of the oil damper for a long period of time, the amount of oil is descreased or the property of oil is changed and as the result, the oil damper may not function satisfactorily.
Another prior art has proposed a countermeasure to cope with such self-oscillation of the suction piston wherein a sliding flange of the suction piston is provided with a pressure controlling port so as to communicate the suction chamber with the ambient air, thereby relieving a direct transmission of sudden change in negative pressure in the venturi portion to the suction chamber. However, according to this prior art, a reciprocating motion of the suction piston may not quickly respond to the change in the negative pressure at acceleration, and accordingly, the response performance of the engine is reduced. In particular, during low intake air flowing stage, the amount of reciprocating motion of the suction piston varies greatly as a result of errors in measurement during manufacture of the pressure controlling port, and thus the air-fuel ratio disadvantageously fluctuates.